1. Field of the Invention
The present invention relates to crash survivable protective enclosures used for flight recorders, cockpit voice recorders and solid state memory devices used in aircraft and other vehicles operating over both land and water. The invention is directed towards a crash survivable protective enclosure which is capable of withstanding high impact forces, shock and mechanical penetration forces that commonly occur in a crash and in addition is capable of withstanding the high temperatures, flames and heat that often develop after a crash. Moreover, the new and improved protective enclosure is especially designed for resisting damage to solid state memory devices even when the protective enclosure is subjected to immersion in salt water for prolonged periods of up to thirty days and for depths of up to 20,000 feet.
In particular, the crash protective enclosure in accordance with the present invention, is adapted to meet or better all of the requirements as set forth in EUROCAE Document No. ED-55, which document sets forth the current requirements for flight recorders utilizing solid state memory devices.
One of these requirements is that the recorded information be retrievable after the protective enclosure has been exposed to sea water at an immersion depth of 20,000 feet for a duration of thirty days. The protective enclosure of the present invention is specifically designed/adapted to conform and meet this requirement in a new and unique way.
2. Background of the Prior Art
Solid state memory devices used for crash protected flight recorders such as EEPROM's or flash memory units are typically packaged in either a ceramic or a plastic protective enclosure. A ceramic package protects the memory devices utilizing a small, hermetically sealed air cavity in which the memory devices are contained. The plastic packages are constructed utilizing a molding process carried out under high molding pressures wherein the device and lead wires extending to the memory device are totally embedded in a plastic compound so that few, if any, air spaces, bubbles or air voids exist within the plastic package immediately surrounding the memory device.
U.S. Pat. No. 4,944,401 describes a crash survivable enclosure capable of surviving relatively high value hydraulic pressures (8,660 psi) that would exist at an immersion depth of 20,000 feet. The crash, fire and water immersion protection is provided by the physical design of the enclosure with emphasis on the high pressure forces encountered during deep and prolonged salt water immersion. Tests have been performed on ceramic enclosed memory devices directly exposed to 20,000 foot depth equivalent pressures in salt water immersion and these tests have shown conclusively that ceramic packages cannot usually support and protect against damage to the recorded information under high hydraulic pressures of this magnitude. In these tests, the ceramic walls generally collapse under such pressures and the memory devices contained within are substantially destroyed. Tests on plastic resin packaged memory devices, however, including both dual in-line as well as PLCC types have been successfully conducted simulating exposure to 20,000 foot equivalent depths of salt water for periods of thirty days. The tests have demonstrated a capability of the plastic enclosed memory devices to survive well in this harsh environment and still yield the vital flight memory data previously recorded.
The use of at least one pressure resisting vessel in a protective enclosure as set forth in U.S. Pat. No. 4,944,401 is thought to be generally desirable for survival of the internal memory devices and the flight data therein when either a ceramic or a plastic enclosed memory device is utilized.